Failure to delegate credentials, SPN problems

[sgrigg]

I get the following error when trying to publish Sharepoint 3.0 using ISA 2006:

ISA Server failed to delegate credentials using Kerberos constrained
delegation to the Web site published by the rule Intra. Check that the SPN:
http/computer.domain.com configured in ISA Server matches the SPN in Active
Directory.

The SPN definately exists and this rule used to work. The only thing I can
think of that has changed is that the domain functional level has changed
from 2003 to 2008R2 but this error could be caused by another change I'm not
aware of.

How do I troubleshoot/fix?

Thanks!